It's a matter of how aggressive is the fog of war and removal of irrelevant entities. Sometimes it's high, sometimes it's low. Too high, and people glitch into existence on high-latency connections. Too low, and you know exactly where people are far behind where you need to.

Many custom servers in games like CS enable a more aggressive version, in cases where you geographically will be close, so you can rule out high latency connections and have more aggressive fog of war.

I believe in the Quake case they made all the walls transparent so you could see everyone coming. Which means they weren’t doing much fog of war at the time.

Regarding the Quake engine and its evolutions (Q3, Call of duty, RTC Wolfenstein), the most common trick was to modify the rendering mode of all player entities so that they are drawn above everything else.

The engine has a flag that is mainly used to draw the player's weapon (which is rendered above the rest). Using this flag for players was the "engine compliant" way of making a wall hack.

You could also replace player model with a big spiky thing. The spikes would poke through walls.

Or in team fortress, replace the sniper's red aimpoint dot with a big model. They become easier to avoid.

Of course not great from large distance / players behind multiple walls but I think the server wouldn't send you the info of those guys anyway.